The present invention is a modification of the conventional commercial system used to extract bitumen from mineable oil sand. In order to understand the manner in which the invention departs from this conventional system and to appreciate the discoveries on which the invention is based, it is first useful to describe the conventional system.
As previously stated, the invention has to do with oil sand, specifically the oil sand of the Athabasca deposit which exists in Northern Alberta. This oil sand comprises sand grains that are water-wet or individually coated with a thin sheath of water. The bitumen or oil is present as flecks located in the interstices between the wet grains.
At applicants' plant, the deposit is surface mined by first removing overburden and then using a dragline to excavate the oil sand and dump it to one side in the form of a windrow. A bucket wheel reclaimer transfers this windrowed oil sand on to the feed end of a conveyor belt train, which carries it to an extraction plant.
Applicant's operation involves mining about 300,000 tons of oil sand per day in this way. Four draglines are employed, each feeding a separate reclaimer and conveyor belt train.
Each such conveyor belt train comprises a plurality of separate endless conveyors placed end to end in series. The conveyors of one train typically can extend a length of 5 miles.
The conveyor system being utilized is characterized by a number of disadvantages, including:
That each conveyor consumes a large amount of electric power. A 72 inch wide conveyor having a length of 3 miles requires several 1200 horsepower motors for operation; PA1 That the conveyor train has to turn corners, which is a difficult and expensive operation requiring use of a multiplicity of short straight conveyors at the turn; PA1 That the tacky bitumen causes some oil sand to adhere to and build up on the belt surface. This creates a dead load which is difficult to prevent and remove; and PA1 That the conveyors are subjected to heavy wear in this service, due to impacts by rocks in the oil sand and the erosive nature of the sand. PA1 heating the viscous bitumen, to reduce its viscosity and render it more amenable to separation from the sand grains; PA1 dispersing the heated bitumen from the solids and into the water; PA1 ablating or disintegrating the normally present lumps of oil sand, so that they will not be lost with oversize rocks in a screening step which immediately follows tumbling; PA1 entraining air bubbles in the slurry; PA1 coalescing some small bitumen flecks into larger flecks to make them amenable to aeration and subsequent separation; and PA1 aerating bitumen flecks by contacting them with air bubbles, whereby the bitumen coats the air bubbles. PA1 the total recovery of bitumen obtained, in the form of the sum of primary and secondary froth, is high; PA1 the loss of bitumen with the tailings is low; and PA1 the bitumen is predominantly recovered in the form of primary froth. PA1 a high total bitumen recovery (e.g. 95%); and PA1 low bitumen losses with the tailings (e.g. 3%). PA1 a low total bitumen recovery (e.g 85%); and PA1 high bitumen losses with the tailings (e.g. 12%). PA1 sufficient bitumen in the oil sand slurry would become properly aerated in a pipeline so as to yield: PA1 That if a slurry, comprising oil sand, water and process aid, is formed so as to entrain air bubbles and is pumped through a pipeline a distance in the order of about 2.5 km (which is commonly less than the distance between the surface mine and the extraction plant), complete conditioning of the slurry is achieved. More particularly, a sufficient quantity of the contained bitumen becomes aerated and is rendered buoyant. As a result, the slurry may be introduced directly into the PSV of a conventional separation circuit, in which PSV spontaneous flotation takes place to yield total recovery, underflow loss, and froth quality values that are comparable to those obtained by a conventional extraction train involving a tumbler and separation circuit; PA1 That the slurry may be at a relatively low temperature (e.g. in the order of 50.degree. C.) and yet conditioning may still be successfully completed as aforesaid; PA1 That there is a "conditioning breakover point" for a particular slurry during the course of passage through a particular pipeline. More particularly, with increasing retention time up to the breakover point, there is: PA1 The breakover point indicators when conditioning is "complete". Such complete conditioning of the slurry is reflected in the total recovery and tailings loss values resulting from subsequent processing of the slurry in a conventional separation circuit. More particularly, the total recovery of bitumen will exceed 90% by weight and the tailings loss of bitumen will be less than 10%, with respect to a feed of sufficient quality to be acceptable for a conventional extraction circuit. Preferably the total recovery of bitumen and bitumen losses for good and poor grade oil sands will be of the order of those values previously stated; PA1 That if the slurry is pumped further through the pipeline after conditioning is complete, significant emulsification does not occur. Stated otherwise, the total recovery and tailings loss values remain generally constant, even though retention time in the pipeline far exceeds that required to complete conditioning; and PA1 That if the completely conditioned slurry is subjected to separation of the coarse solids (as by settling) part way along its passage through the pipeline, it is found that the solids will readily separate in a substantially clean condition. Stated otherwise, once completely conditioned, passage of the slurry through the pipeline may be interrupted and the coarse solids may be separated without appreciable bitumen loss. The remaining slurry may then be pumped through the pipeline the remainder of the distance to the extraction plant.
In summary, the conveyor systems used are a troublesome and expensive means for transferring the oil sand from the mine to the extraction plant.
It will also be noted that a conveyor system transports the whole oil sand to the plant, for the sole purpose of extracting the bitumen. The bitumen constitutes only about 6-15% by weight of the processable oil sand mass. Conveying all of the associated gangue material significantly reduces the economic attractiveness of the operation.
Once the oil sand arrives at assignees' extraction plant, it is fed into one of four extraction circuits, each of which begins with a tumbler. These tumblers are large, horizontal, rotating drums. In the drum, the oil sand is mixed with hot water and a small amount of process aid, normally sodium hydroxide. Steam is sparged into the formed slurry as it proceeds down the length of the slightly inclined drum. In greater detail, each drum is 30.5 m long and 5.5 m in diameter. Each such drum is fed about 4500 tph of oil sand, 1100 tph of hot water (95.degree. C.) and 5 tph of aqueous 10% caustic solution. Steam is injected into the slurry, as required, to ensure a final slurry temperature of about 80.degree. C. The retention time in the drum is about 3 minutes.
The process in the tumbler seeks to attain several ends, namely:
The expression, used in the industry to identify the sum total of these various actions, is "conditioning" the slurry. A definition is given below with respect to when conditioning is "complete" for the purposes of this invention.
After being conditioned in the tumbler, the slurry is screened, to reject oversize, and simultaneously is diluted with additional hot water to produce a slurry having about 50% solids by mass (based on the initial oil sand feed).
The screened, diluted slurry is fed into a large, thickener-like vessel referred to as a gravity separation vessel or primary separation vessel (or "PSV"). The vessel is open-topped, having a cylindrical upper section and a conical lower section equipped with a bottom outlet. The diluted slurry is temporarily retained in the PSV for about 45 minutes in a quiescent state. The coarse solids sink (having a density of about 2.65), are concentrated in the cone, and exit through the bottom outlet as a fairly dense tailings stream. The non-aerated bitumen flecks have a density of about 1.0 and thus have little natural tendency to rise. However, the bitumen has an affinity for air. Because of this property, some of the non-aerated bitumen flecks form films around the air bubbles present in the slurry and join with the aerated bitumen created in the tumbler in rising to form bitumen froth at the surface of the slurry. This froth overflows the upper lip of the vessel into a launder and is recovered. The froth recovered in this manner is referred to as "primary bitumen froth". The process conducted in the PSV may be referred to as involving "spontaneous flotation".
The watery suspension remaining in the central portion of the PSV contains some residual bitumen. Much of this bitumen was not sufficiently aerated so as to be recovered as primary froth from the PSV. Therefore it is necessary to further process this fluid to recover the remaining bitumen. This is done by means of vigorously sub-aerating and agitating the fluid in one or more secondary recovery vessels. For example, a dragstream of the middlings from the PSV may be fed to a series of sub-aerated flotation cells. A yield of bitumen froth, termed secondary froth, is recovered. Flotation in the PSV may be referred to as "spontaneous flotation" while flotation in the secondary recovery vessels may be referred to as "forced air flotation".
The combination of the PSV and the subsequent secondary recovery means is referred to herein as the "separation circuit".
The primary bitumen froth is formed under quiescent condition and hence has less entrainment of gangue material. Thus it is considerably "cleaner" than secondary froth, in that it contains less water and solids contaminants. So it is desirable to produce the bitumen in the form of primary froth, to the greatest extent possible.
If conditioning has been properly accomplished, the following desirable results are achieved:
At this point it is appropriate to make the point that the nature of the oil sand being processed has a marked influence on the results that are obtained. If the oil sand is of "good" grade (i.e. high in bitumen content--e.g. 13.2% by weight--and low in--325 mesh solids--e.g. 15% by weight) it will process well, giving:
If the oil sand is of "poor" grade (i.e. low in bitumen content (e.g. 8%) and high in fines content (e.g. 30%), it will process relatively poorly, giving:
In summary then, the conventional extraction circuit comprises a tumbling step that is designed to condition the slurry. Tumbling is followed by a sequence of spontaneous and forced air flotation steps. If conditioning is properly conducted, the total bitumen recovery and bitumen loss values for different grades of feed will approximate those illustrative values just given.
Now, it has long been commonly known that particulate solids may be slurried in water and conveyed by pumping them through a pipeline, as an alternative to using conveyor belt systems.
However, to the best of our knowledge the public prior art is silent on whether oil sands can successfully be conveyed in this fashion, as part of an integrated recovery process. More particularly, the literature does not teach what would occur in such an operation.
The present invention arose from an experimental project directed toward investigating pipeline conveying of oil sands in aqueous slurry form.
The project was carried out because it was hoped that pipelining a slurry of oil sand might prove to be a viable substitute for the conveyor belt plus tumbler system previously used to feed the separation circuit. There were questions that needed to be answered to establish this viability. The answers to these questions were not predictable. More particularly, it was questionable whether:
a high total bitumen recovery, and PA2 a high primary oil froth recovery; or the bitumen would become excessively emulsified in the course of being pumped a long distance through a pipeline, so that the bitumen would become difficult to recover from the slurry. PA2 an increase in subsequent total bitumen recovery from the separation circuit, and PA2 a diminishment in subsequent losses of bitumen with the underflow tailings from the separation circuit.